The invention pertains to silica filled moldable polymers. In particular, the invention pertains to injection moldable polymers wherein the filler is processed into the polymer blends such that the application rate of the blend is significantly increased. More particularly the invention pertains to silica filled moldable polymers where the application rate of the polymer blend has been increased by reducing the average aggregate size of the silica aggregates comprising the silica filler.
Known liquid injection moldable polymers usually employ a polymer composition, one or more fillers, a catalyst and an optional cross-linking agent. The liquid injection polymer composition may be injection molded to form a variety of small parts.
Large injection molded parts are desired. However, it has been found difficult to mold large parts because the molding material has a tendency to cure before the mold is filled. One cause of this problem appears to be traceable to the properties of the filler material. A commonly used filler material is fumed silica that has various structural characteristics. At one level, fumed silica comprises individual sub micron size spherical particles. These particles tend to aggregate into dendritic structures having long radial arms. The aggregates tend to agglomerate into groups or clumps. At sufficiently small inter aggregate distances or at sufficiently high concentrations of the aggregates, the long arms of the dendritic structures begin to interlock and hinder the flow of the mixture containing such interlocked aggregates.
The application rate of materials incorporating fumed silica thus appears to be a function of the average aggregate size. In particular, the length of the radial arms of the dendritically shaped aggregates appear to affect the application rate and physical properties.
U.S. Pat. No. 3,953,487 discloses a method for preparing hydrophobic SiO.sub.2 by reacting solid SiO.sub.2 of a fine aggregate size with organosilicon compounds while subjecting the reaction mixture to mechanical impact, resulting in comminution of the SiO.sub.2 aggregates and agglomerates thereof. In a particular example, SiO.sub.2 aggregates are subjected to a ball milling operation to reduce the average aggregate size. The material in the '487 patent may be combined with polymers of the type described in the above-identified application resulting in liquid injection moldable polymers with acceptable application rates. However, the fumed silica material processed in accordance with the '487 patent is difficult to manufacture requiring many hours of ball milling to achieve the lower aggregate size required. The process is unattractive from a manufacturing point of view because of the time required to prepare the silica in a finely divided state, dust and associated contamination problems. These considerations render the material expensive to purchase.
It is therefore desirable to find an effective method to reduce the average aggregate size of finely divided silica by an amount sufficient to achieve high application rites in polymer compositions having satisfactory physical properties.